Prior art devices exist which fix screws nd other threaded members in a substrate such as an agglomerated material including concrete, plaster and epoxy. Two such screw fixing devices are disclosed in U.S. Pat. Nos. 4,085,652 and 5,085,547. The prior art screw fixing devices include a sheath into the substrate and into which the screw is threaded.
While the sheaths of the above-referenced devices enable the screw to be secured in the substrate, such scew fixing devices have some deficiencies. U.S. Pat. No. 4,085,652, for example, discloses an anchoring device with an overmolded sheath FIG. 1, which has circular ribs and four axial ribs. However, this design of the overmolded sheath makes the screw impossible to remove.
U.S. Pat. No. 5,085,547 improves the screw fixing device by providing a plastic sheath, FIG. 2, into which a screw is threaded and adds a metal sheath surrounding the plastic sheath. The metal sheath is embedded in the substrate and has longitudinal ribs which prevent the sheath from rotating when the screw is threaded into the plastic sheath (FIG. 2). The design of the sheath in the previous screw fixing devices having two longitudinal ribs which are positioned opposite each other is disadvantageous. The longitudinal ribs can create fragile zones 1, Prior Art FIG. 1, in a substrate such as concrete where moisture and stresses on the screw may cause cracking in the substrate. Because the longitudinal ribs lie in the same plane, the fragile zones in the substrate also lie in the same plane and the forces acting on the screw and sheath will be concentrated in that plane causing potentially serious substrate cracking.
Furthermore, the longitudinal ribs of the screw fixing devices extend too close to the top and bottom of the screw fixing sheath. Having the longitudinal ribs close to the top of the screw fixing sheath causes fragile zones in the weaker top layer of the substrate which is more susceptible to fracturing by forces applied to the screw. Having the ribs extend to the bottom of the sheath below the end of the screw also causes fragile zones at the bottom area of the sheath.
Another problem with previous screw fixing devices involved the contamination of the interior of the sheath with water and other debris. During manufacturing and shipping of the screw fixing sheath, water and debris could enter the threaded interior of the sheath causing problems when a screw is threaded into the sheath. Moreover, the accumulation of water and/or ice in the bottom of the sheath after the sheath has already been embedded in a substrate creates even more serious problems. The water and/or ice inhibits the screw or bolt from being completely threaded into the sheath and causes cracking in the sheath and surrounding substrate when tightened. A further contamination problem occurs after the screw or bolt has been threaded into the sheath when water can seep in around the screw or bott threads causing corrosion and damage from expansion when the water freezes.
Another problem occurs when screw fixing devices having plastic sheaths are used in pre-stressed concrete, such as railroad ties. Typically, concrete is pre-stressed by tensioning steel strands tightly in a mold, pouring concrete into the mold around the anchoring sheaths, and cutting the ends of the steel strands after the concrete has begun to set. When cut, the strands constrict and pull inwardly, causing the concrete around the anchoring sheath to exert compression stresses on the sheath. As a result of the stresses in the pre-stressed concrete and as a result of the anchoring sheath not having a modulus of elasticity which closely matches that of concrete, the flexible top region of the prior art anchoring sheath will flex or contract, thereby causing fractures in the concrete around the anchoring sheath.
Accordingly, what is needed is a screw fixing or anchoring device having a sheath which does not create fractures and fragile zones in the substrate where forces may be concentrated in a plane, causing the substrate to fracture.